The invention relates to a method and a circuit arrangement for receiving and evaluating the light output of a light emitting diode of an optical monitoring unit.
Optical monitoring units which operate on the reflection principle and which include a transmitting unit for transmitting light to an object and a physically remote receiving unit for receiving the light reflected from the object, are known, for instance from DE 196 20 147, DE 197 04 793, EP 0 312 788 A2 or EP 0 112 498 A2, and are used for a variety of applications such as, for example, measurement of visibility range, optical seat occupation detection in motor vehicles, reflected light barriers and the like. The light output to be transmitted is determined by means of the diode current and stabilized through a control loop for keeping the diode current constant. The actual light output of the transmitting diodes depends, however, not only on their diode current but also on other influencing quantities, especially degradation and manufacturing tolerances. The light output is important for optical monitoring units because the if, light components reflected from the object must be noticeably emphasized relative to the unavoidable interference light, especially ambient light. To some extent also, a certain proportion of the emitted light is used to assess other quantities, for instance the distance of the object or the contamination in the beam path, for instance on a translucent cover. Even if a component of light reflected from a translucent cover within a housing of the monitoring unit as the result of contamination is guided back to the receiving unit, as proposed in DE 197 04 793, it is quite possible that this light component, which is interpreted there as a contamination signal, might not be influenced by a fluctuating transmitted light output. The light reflected from the object to be monitored is also not suitable for the purposes of detection of light output variations, because the object range to be monitored can change.
Light output measurement involving additional measuring arrangements with further light-sensitive components cannot be considered for cost reasons.
It is an object of the invention to avoid or overcome the above problems of the prior art. According to the invention, a simple method is specified for detecting the light output of a light emitting diode without using additional light-sensitive components. Furthermore, a circuit arrangement for. performing the method according to the invention is extremely simple and permits, in particular, the integration of the transmitting diodes together with this circuit arrangement on a semiconductor substrate. Finally, two particularly preferred possible applications of the method according to the invention are presented.
The invention is based on the known property of all light-emitting diodes that when the polarity is reversed the diodes are light-sensitive at least to a small extent and at least for a specific wavelength range, and generate a photoelectric current or a photoelectric voltage across a resistor. Initially, each light-emitting diode emits light in the forward direction if the voltage is sufficiently high. In this form, diodes are generally used as transmitting diodes and are optimized according to this function in terms of material selection and design. Under appropriately intensive light influence of suitable wavelength being received by the diode, however, every light-emitting diode also generates a photoelectric current flowing in the reverse direction. Apart from light-emitting diodes, components with a physically comparable function, laser diodes for example, can transmit active optical fields or the like. In the article by Mr. Mathias Handwerker in Funkschau 20/1984, p. 83 ff., a reflected light barrier is described, for example, in which two LEDs are used in a housing where one of these LEDs serves as a receiver for the light reflected outside. Reference is made here already to the relatively high photoelectric voltage that can be achieved with certain LEDs and is comparable with photodiodes. Crosstalk of the light to be transmitted automatically occurs here onto the receiver; this is suppressed by appropriate circuit devices.
It is precisely this optical crosstalk between diodes however, that can be utilized in order to detect the light output of a given diode in an optical monitoring unit with a plurality of transmitting diodes in a transmitter unit. For detecting the light power, only one of the diodes is operated as a transmitting diode to emit light at any one time, while at least one other diode is connected. as a receiver to receive the light component reflected inside the transmitting units, which is recorded as a signal that is proportional to the light output. The light output is derived from this.
With all light-emitting diodes, it is possible to detect the light output with at least sufficient accuracy to decide whether light is still being emitted at all, because the crosstalk is relatively powerful compared with influences due to ambient stray light. Precisely because of this direct coupling, the intensity of the reflected light component also can be distinguished for a wide range of light power with the simplest of circuit devices. This form of measurement is in any case considerably more accurate and reliable than derivation from the current flow.
Naturally, it is only important for measuring the light output that only one of the transmitting diodes transmits in order to void influences from the other diodes on the receiver. However, it is sufficient to perform measurement of the light output at certain intervals, and in normal operation all transmitting diodes can be operated as such, also at the same time where applicable.
It is particularly preferable to use the diode immediately adjacent to the transmitting diode as the receiver because the optical crosstalk is particularly high here, and also because no allowance need be made for the distance between the active transmitting diode and the adjacent diode used as the receiver. For this method, a particularly preferred circuit arrangement results, in which two transmitting diodes at a time are connected together in pairs.